Peptide is prepared by hydrolysis of protein with acid or enzyme treatment, and it is known to show excellent digestion and absorption properties when ingested orally. Peptide is used in dietary supplements and sports drinks as low-allergic nitrogen sources. Recently, a peptide has been attracting attention as functional materials, since it has been verified to have beneficial effects, such as alleviating muscle fatigue, reducing blood cholesterol levels, lowering blood pressure, and stimulating microbial fermentation.
High degree of clarity is one of the quality requirements for developing the commercial availability of peptide in dietary supplements, including liquid food, sports drinks, and culture media markets. Low clarity causes reduction in marketability as food and difficulty in isolating useful components from medium.
A variety of raw material proteins for preparing peptide have been developed, including animal proteins contained in egg white, milk, fish meat and the like; plant proteins contained in soybean, wheat and the like; and microbial proteins. Among them, for example, peptide prepared using soy protein as raw material has been developed, including SPI peptide prepared from soy protein isolate as raw material, defatted soymilk peptide prepared from soymilk, which is derived from defatted soybean, as a raw material, and soymilk peptide prepared from soymilk, which is derived from whole fat soybean, as a raw material. While soymilk peptide is poor in nitrogen content as compared with SPI peptide, it contains soy oligosaccharides and minerals in a balanced manner. In particular, soymilk peptide is widely developed in the medium market and in dietary supplements.
During the process of producing peptide from a variety of soy protein, insoluble matter, which is thought to be a polymerization product of decomposed peptide, is known to be generated. Therefore, in general practice, such insoluble matter is removed by filtration, such as diatomite filtration, microfiltration and ultrafiltration, in the preparation of high clarity peptide. However, defatted soymilk peptide, in particular, has very poor filtering properties as compared with SPI peptide, and it causes an increase in production costs. Therefore, improvement of the properties has been longed.
Meanwhile, calcium is known as a highly reactive divalent cation, and various applications as a flocculant have been proposed. By way of an example of application of calcium to peptide, Patent Document 1 proposes a method for eliminating phytic acid by adding calcium to SPI peptide prepared using a soy protein isolate (SPI) as a raw material. However, in Patent Document 1, calcium is used for the purpose of reducing the formation of residues from phytic acid, and it makes no particular mention of the influence of calcium on the filtering properties of SPI peptide.
Patent Document 2 describes calcium content in relation to defatted soymilk peptides. However, the description relates to the determination of endogenous calcium levels in soymilk, and no idea of improving the filtering properties of defatted soymilk peptide by artificially adding calcium is found in Patent Document 2.